Nitrogen retention in two Norwegian watercourses of different trophic status

Nitrogen retention in waterbodies from the mesotrophic/eutrophic Eikeren watercourse and the oligotrophic Bjerkreim watercourse was estimated by mass-balance studies over a period of three years. Studies of lakes of different trophic status revealed that the total annual nitrogen retention was higher in eutrophic lakes (20-30%) than in oligotrophic lakes (< 5%). In contrary to other studies, the N retention seemed not to be associated with water residence time. These findings indicate that N retention is bound to biological processes and not to passive sedimentation processes. A high N:P ratio in most of the lakes indicates that only part of the N pool in the lakes is involved in the biological processes leading to N retention. Thus, we suggest that N retention in Norwegian lakes seems to be phosphorus limited, i.e. high N retention is not to be expected unless the N:P ratio is close to the proportional needs for the aquatic plants.